Introduction

The benefits of regular physical activity outweigh the inherent risk of
adverse events. Still, adverse events are common even if usually not severe and
are an impediment to widespread participation in regular physical activity (1-4). Awareness of the types and causes of activity-associated
adverse events can be helpful. Selection of low risk activities and prudent
behavior while doing any activity can minimize the frequency and severity of
adverse events and maximize the benefits of regular physical activity.

Physical activity-related adverse events are undesired health events
that occur because a person is physically active. They may be mild or severe
and include such diverse maladies as musculoskeletal injuries, cardiac
arrhythmias, heat injuries, and infectious diseases. Musculoskeletal injuries
are the most common type of physical-activity associated adverse event, have
generated the most scientific study, and are the primary focus of this chapter.
Musculoskeletal injuries may be sudden, such as a torn anterior cruciate
ligament in the knee in a soccer player, or slow to develop, such as pain
around the knee due to the iliotibial band syndrome in a runner or hiker. The
current scientific literature does not routinely distinguish between these two
types of injuries, sometimes referred to as traumatic and overuse, despite the
value of doing so.

The chapter also considers in detail sudden adverse cardiac events
because they may result in severe outcomes (e.g., death, myocardial
infarction), fear of them likely reduces participation in activity, and their
occurrence can be reduced. Other types of adverse events (e.g., heat-related
illness, infections) are mentioned largely to provide examples and maintain
awareness of the broad array of potential adverse events.

The factors that increase susceptibility to these adverse events also
are diverse, and include the type of activity being performed (e.g., walking
versus rugby), the dose of activity (the amount, as determined by the
frequency, duration, and intensity), personal characteristics (e.g., age,
physical activity habits), equipment or protective gear used (e.g., bike
helmets), and environmental conditions (e.g., proximity to traffic, weather)
(Table G10.1).

Whenever possible, the chapter draws from studies of the general
population rather than research on elite or competitive athletes. Much of the
research in this area has focused on competitive athletes. The frequency,
duration, and intensity of the exposure and, sometimes, even the rules of the
game differ markedly between competitive athletes and the general population,
making extrapolation from the injury experiences of competitive athletes to the
general population likely to be misleading.

While considering both the spectrum of adverse events and their causes,
the chapter emphasizes the key factors of any physical activity program:

The type (mode) of activity;

The dose of activity as determined by the frequency, duration, and
intensity of participation; and

These 3 factors are largely under individual control, and though the
exact proportion of adverse events attributable to decisions in just these
areas is not known, it is likely substantial. The risk of injury varies widely
for different types of activities, with low-impact activities, such as walking
or gardening, associated with the fewest musculoskeletal injuries. A higher
dose of activity, especially among those who have previously not been active,
is associated with more musculoskeletal injuries. Finally, modest and gradual
increments in frequency and duration of activity are preferred at the beginning
of any effort to increase aerobic activity. Augmentation of intensity, if
desired, should come later (6). Attaining the desired level
of activity may require a year, especially for elderly, obese, or habitually
sedentary individuals (7).

Although the risk of activity-related injury is greater among persons
who are more active, the risk of other types of injuries (e.g., motor vehicle,
work-related) may be less, making the overall risk of injury for active people
no greater than that for sedentary people. Only two population-based studies
have examined this issue. One reported that people who ran or participated in
sports activities were about 50% more likely to report an injury
(activity-related or not) than people who reported walking for exercise or were
sedentary (8). The other reported no significant
differences in overall injury rates (activity-related or not) between inactive
people, irregularly active people, and people who met current recommendations
for physical activity (9). More studies of this type are
needed, but it is possible that regular physical activity may cause some
injuries and prevent others, and that physically active people may have no more
injuries than sedentary individuals.

Review of the Science

Overview of the Questions Asked

The 5 major questions addressed in this chapter are:

What types of activities have the lowest risk of musculoskeletal
injuries?

How does the dose of physical activity affect the risk of
musculoskeletal injury?

Are individuals at increased risk of sudden adverse cardiac events
when they are being physically active?

What general factors influence the risks of musculoskeletal injury
and other adverse events related to physical activity?

Do the benefits of regular physical activity outweigh the
risks?

Data Sources and Process Used To Answer
Questions

The Adverse Events subcommittee used the Physical Activity
Guidelines for Americans Scientific Database developed for the PAGAC
process for this chapter (see Part F:
Scientific Literature Search Methodology, for a detailed
description of the Database). The diversity of exposures, mediators, and
outcomes plus the limited number of recently published papers on several
important topics required an expanded search for pertinent work. The
subcommittee therefore conducted special literature searches on upper
respiratory infections and medical expenditures and added these publications to
the Scientific Database. A literature search pertaining to air pollution and
health was conducted but not added to the Database because physical activity
was not a required component of the search. Additional citations were drawn
from the Institute of Medicine's summary report of the Workshop on Adequacy of
Evidence for Physical Activity Guidelines Development (10), review articles, consultants' recommendations, and other
citations in pertinent articles.

Question 1. What Types of Activities Have
the Lowest Risk of Musculoskeletal Injuries?

Conclusions

Activities with fewer and less forceful contact with other people or
objects have appreciably lower injury rates than do collision or contact
sports. Walking for exercise, gardening or yard work, bicycling or exercise
cycling, dancing, swimming, and golf, already popular in the United States, are
activities with the lowest injury rates.

Rationale

Risk of musculoskeletal injury varies substantially across different
activities and is determined to a large extent by the frequency and force of
collisions or contact with other people, the ground, or other inanimate
objects. Categorization of activities by risk is difficult because style and
rules of play vary by age, location, and other factors. However, the Committee
on Sports Medicine and Fitness has proposed general categories that provide a
guide to the injurious forces associated with specific activities (11). In collision sports (e.g., football, ice hockey,
wrestling) participants purposefully hit or collide with each other or
inanimate objects. In contact sports (e.g., basketball, soccer) participants
make contact with each other but usually with less force. In limited-contact
sports (e.g., baseball, ultimate Frisbee) participants' contact with other
players or objects is infrequent or unintentional. In non-contact sports (e.g.,
running, swimming, tennis) contact between participants is uncommon. In
general, the risk of injury is higher for collision or contact sports than for
limited- or non-contact activities.

What Types of Activities Are Associated With the Lowest Risk of Injury
in the General Population?

Most of the published information about the number of injuries incurred
during different types of activities does not take into account either the
number of people participating in various activities or the length of time they
spend doing so. As a result, tabulations of injuries seen in emergency
departments or other health care settings usefully describe the load on medical
care services but do not allow risk estimates for different activities at the
individual level. Very few studies have provided injury rates based upon the
amount of time spent doing the activities.

A survey of injury risk in the general population in Finland reported a
wide range of injury risk for different types of physical activities (Table G10.2[1]) (12).

Table G10.2. Injuries per 1,000 Hours of
Participation and Per 1,000 Participants by Activity, Finland (12)

Reported activity-related injuries per 1,000 hours of participation
ranged from 0.2 for walking as a commuting activity to 18.3 for squash. Injury
rates were lower for commuting activities (range 0.2 to 0.5 per 1,000 hours of
participation), lifestyle activities (range 0.33 to 1.01), and noncontact
sports (range 0.3 to 4.7, median 1.6) than were the rates for limited contact
sports (range 2.0 to 18.3, median 4.1) and collision and contact sports (range
6.7 to 9.1, median 7.8). The findings are based on a year long population-based
random survey of Finns aged 15 to 74 years, 92% of whom agreed to record all
physical activity sessions of 15 or more minutes and register all acute and
overuse injuries that "caused a significant complaint to the subject" and that
were related to the activities. Participation rates and related injuries were
reported by telephone once every 4 months. It is interesting to note that
injury rates for walking and cycling during commuting activities (0.2, 0.5,
respectively) were one-sixth and one-fourth the rates for walking and cycling
performed as sports or recreation (1.2, 2.0, respectively). This, too,
indicates that the same activity done for different purposes and with different
frequency, duration, and intensity leads to different injury rates.

Four other surveys of the general population also report appreciably
lower injury rates among participants of non-contact activities such as
walking, bicycling, gardening, golf, or swimming (14-17).
The surveys, conducted in the United States, Canada, and Australia (2 surveys),
report injury rates that are not directly comparable because of differing
definitions of injury (e.g., medically attended versus any injury), differing
time periods (e.g., 2 weeks versus 1 year), and the inclusion of different
activities. Regardless, the relative safety of non-contact activities when
compared to limited-contact, contact, and collision activities is present in
all reports. In the 3 surveys in which activity-specific participation rates
were provided, walking was the most commonly reported activity, generally by a
substantial amount (14;16;17).

What Types of Sports Have the Lowest Rates of Injury for Children,
Youth, and Young Adults?

Surveys of injuries among college sports (18;19), high school sports (20), and
community organized sports leagues (21) have been
conducted (Table G10.3).

The methods within each survey enable comparison of injury rates among
different sports within the age-group of interest (i.e., collegiate athletes,
high school athletes, or children aged 7 to 13 years in organized community
sports leagues) and similar enough to allow an examination of general patterns.
In all surveys, an athlete participating in a practice or game was an
"athlete-exposure." From 1988-1989 through 2003-2004 the National Collegiate
Athletic Association used standard methods to monitor the incidence of injury
in 15 men's and women's collegiate sports (18;19). Each year approximately 250 schools voluntarily
participated in the surveillance system. Injuries were counted if they occurred
as a result of participation in practice or game, required medical attention by
a team athletic trainer, and resulted in restricted participation for one or
more days. From 1995 through 1997 data were collected from 250 athletic
trainers working directly with high school sports programs on a daily basis (20). Injuries were counted if they caused cessation or
participation on the day of or day after the onset; in addition, all fractures
or dental injuries were counted. From 1999 to 2000, injury reports were
collected from coaches in the community leagues for children aged 7 to 13 years
in Pittsburgh (21). Injuries were counted if a coach came
onto the field to check the condition of a player, if a player was removed from
participation, or if a player needed any type of first aid during an event.

In the surveys of college and high school athletes, injuries were less
common in limited contact sports than collision or contact sports. Among
children's community leagues, injury rates were higher for football than for
baseball, softball, or soccer, but the differences between sports were less
apparent than for the older youths. None of the surveys included non-contact
sports such as swimming or golf. In all 3 surveys, injuries were less common in
practice than games.

Question 2. How Does the Dose of Physical
Activity Affect the Risk of Musculoskeletal Injury?

Conclusions

The risk of activity-related injury (but not necessarily overall injury)
is directly related to a person's usual amount of physical activity. Research
with a variety of populations and methods also shows that when individuals
increase their usual amount of physical activity the risk of injury is related
to the size of the increase. A series of small increments in physical activity
each followed by a period of adaptation is associated with lower rates of
musculoskeletal injuries than is an abrupt increase to the same final
level.

Fewer studies are available to examine the independent contributions to
injuries of the components of physical activity  frequency, duration,
intensity. However, available research indicates that each component is a
contributory factor and that the composite amount is a more important
determinant of risk than any component by itself.

Currently available information suggests that the commonly recommended
level of regular physical activity, about 500 metabolic equivalent
(MET)-minutes per week, has a low (but not precisely measured) rate of
associated musculoskeletal injury. However, little information has been
reported about the risks of injury at this level.

Rationale

The dose of activity is determined by its frequency, duration, and
intensity. Both dose and change in dose are important determinants of
musculoskeletal injuries and, to a large extent, are under personal control.
This is especially important because many Americans are inactive and should be
encouraged to increase the amount of physical activity in which they engage.
For discussion purposes, studies providing insight into the relationship
between changes in dose of activity and risk of injury can be grouped into 3
categories:

People who have self-selected their current activity program;

People with previously differing levels of activity who all adopt
the same activity program simultaneously (e.g., military recruits); and

People with previously similar levels of activity who are assigned
different but higher levels of activity (e.g., intervention research).

Evidence from each of these 3 groups of studies indicates that: 1) the
amount of activity is directly related to the risk of musculoskeletal injury
and 2) the change in amount of activity is directly related to risk of
musculoskeletal injury. Military recruits who are the least fit or least
physically active before basic training and experimental subjects who are
assigned higher amounts of activity are the most likely to become injured.
Stated another way, the same amount of new activity is more likely to cause
injury in sedentary individuals than in active individuals. These findings
suggest that a series of small increments in physical activity each followed by
a period of adaptation is associated with lower rates of musculoskeletal
injuries than is an abrupt increase to the same final level. Previous physical
activity recommendations for children (22), adults (23), and older adults (24) have
suggested gradual augmentation of activity levels to prevent injuries and
improve adherence.

What Is the Relationship Between Self-Selected Doses of Activity and
Risk of Injury?

The clearest evidence of the direct relationship between dose of
activity and risk of injury comes from 7 studies of running injuries.
Individuals who run 40 miles per week or more (4,000 or more MET-minutes per
week) [2] are 2 to 3 times more
likely to have had a running injury in the past 12 months than are individuals
who run 5 to 10 miles per week (500-1,000 MET-minutes per week) (3;8;25-29) (Figure G10.1).

Five studies were retrospective cohort studies and 2 were prospective
cohort studies. Injuries in all studies were self-reported but definitions
varied. Three counted symptoms that caused the runner to modify his or her
usual program, take medication, or seek medical advice (3;27;29), 2 counted
symptoms recognized by the participant as an injury (8;28), 1 counted only injuries for which medical care was
sought (26), and 1 counted only injuries that caused the
runner to stop running for at least 7 days (25). Despite
varying definitions, all studies reported increases in the risk of injury as
the number of miles per week increased. For those studies providing information
separately about injuries for which medical attention was sought, the findings
were similar (3;26;28). The trends were similar for males and females and for
runners of different ages. No information about race or ethnicity is reported
by any of these studies. Similar findings have been reported for triathletes,
another group for which estimating the dose of activity is relatively
straightforward (30;31).

Figure G10.1. Percentage of Recreational
Runners or Walkers Injured by Average Number of Miles Run per Week

What Are the Relative Importances of Frequency, Duration, and Intensity
to the Risk of Musculoskeletal Injury?

Although the evidence is limited, in these studies of runners the total
dose of running appears to be more important than any of its components. Among
the studies of running injuries, only 2 reported information about the
relationship between frequency and injury risk. One reported that greater
frequency (number of days per week of running) was significantly related to
injury even after total dose (i.e., miles per week) and other factors were
taken into account (29); the other reported that the
relationship was not significant after adjustment for other factors, including
total amount (27). Two studies reported on the
relationship between duration of episodes and injury. One reported a
significant relationship between minutes of running per week and injury even
after other factors were taken into account (8); the other,
in a multivariate analysis, reported an association between the distance of the
longest run per week and injury (29). None of the 4
studies that examined the usual speed of running observed a relationship after
total amount and other factors were taken into account (3;25;28;29). Two of these studies, however, reported that competitive
runners were more likely to be injured than recreational or noncompetitive
runners (28;29). Similar findings
have been reported for competitive versus club athletes in other sports (31;32). Competitiveness may be a
surrogate for relative intensity, suggesting that the athletes at greatest risk
of injury are those performing near the top of their capacity.

Is the Risk of Injury per Mile Equivalent for Runners With Different
Weekly Mileages?

Although the risk of injury is directly related to the volume of
activity, the risk per unit of exposure appears to diminish as the volume of
exposure increases. Among runners, the risk of injury per mile is about 10-fold
higher at 5 miles per week (500 MET-minutes per week) than 40 miles per week
(4,000 MET-minutes per week) (28;33).
A similar observation has been made for subjects in the Aerobics Center
Longitudinal Study (34). The annual risk of injury for
persons expending about 2,000 kilocalories per week (1,632 MET-minutes per
week) [3] in exercise was 22% or
about 11% per 1,000 kilocalories per kilogram (816 METminutes per week)
of exercise; the annual risk for persons expending 10,000 kilocalories per week
(8,160 MET-minutes per week) was 65% or about 6.5% per 1,000 kilocalories (816
MET-minutes). These findings are consistent with the suggestion that any given
increase in volume of activity (e.g., adding 500 MET-minutes per week of
activity) produces a greater increase in risk of injury for those who are less
active than it does for those who are more active.

What Is the Relation Between an Assigned Dose of Physical Activity and
Injury Among Persons of Different Levels of Fitness or Physical Activity
Habits?

Military recruits are young healthy adults who undergo 2 to 3 months of
rigorous, often vigorous, aerobic and muscular training, primarily running,
marching, drill, and general conditioning exercises. Typically, recruits have 5
to 6 days of activity per week with, on average, 40 minutes of running or
marching, 10 minutes of drill (learning to march in unison), 10 minutes of
general condition (calisthenics), and 10 minutes of stretching per day, for a
weekly total of about 2,725 to 3,270 MET-minutes (35).
This level of physical activity is approximately 6 times the currently
recommended amount for the population (36). Recruits
experience high levels of overuse musculoskeletal injuries during this period
(11% to 37% for men, 22% to 67% for women) (37). The
symptomatic onset of injuries corresponds closely to the dose of activity (38) (Figure G10.2).

What Factors Are Most Consistently Associated With High Rates of
Musculoskeletal Injuries During Basic Training?

Low levels of aerobic fitness and prior physical activity are 2 of the
most consistently observed risk factors for injuries among the recruits. Six
studies have demonstrated an inverse relationship between measures of aerobic
fitness and risk of injury (35;39-43)
(Figure G10.3). Aerobic fitness was determined by
timed distance runs ranging from 0.75 to 2.0 miles or, in one case, peak
VO2 (40). Outcomes included time loss injuries
(39;40), lower extremity injuries (35), and stress fractures (41-43).
Participants in the studies were either all male, all female, or stratified by
sex.

These same studies have reported similar findings for a variety of
measures of pre-training physical activity practices, including frequency of
running, frequency of physical activity producing a sweat, frequency of
physical activity in general, self-assessed physical activity levels, and
self-assessed relative fitness level (35;39-43). Similar findings have been reported when the recruits
have been followed beyond the initial basic training period (44;45). Students in physical education
classes (46) and participants in aerobic dance classes (47) also have been shown to be more likely to be injured if
they are less physically active outside of class.

Modifications of the basic training program (e.g., less running) (48;49) or providing a formal
pre-training program to recruits with low fitness (50)
have resulted in reductions in the incidence of injuries among recruits in
basic training while maintaining fitness goals (51).

Figure G10.2. Hours of Drill per Marching
Plus General Conditioning (Solid Line, Left Axis) and Injuries per 100 Recruits
(Dotted Line, Right Axis) by Week of Training

Figure G10.2. Data Points

Weeks of Training

Injuries per 100 recruits

March+conditioning

1

4.0

17.0

2

7.4

10.1

3

7.0

10.0

4

3.3

5.8

5

1.3

3.3

6

3.2

1.0

7

3.0

3.6

8

7.2

14.2

9

1.2

4.5

10

8.4

14.8

11

4.8

11.8

12

0.6

1.6

Figure G10.3. Rate or Odds Ratio for Injury
Among Military Recruits During Basic Training by Aerobic Fitness at Entry

Are These Findings Consistent With Principles of Exercise Training
Programs?

The findings in military recruits, students, and runners are consistent
with the 2 major principles of exercise training programs: 1) overload and
adaptation, and 2) specificity of response (52). The
overload and adaptation principle states that function is improved when tissues
(e.g., muscles) and organs (e.g., heart) are exposed to an overload (i.e., a
stimulus greater than usual) and provided time to recover and adapt. Repeated
exposures to a tolerable overload are followed by adaptation of the tissues and
organs to the new load and improvements in performance and function. Too large
an overload or insufficient time for adaptation, however, leads to injury and
malfunction. The principle of specificity states that the adaptation and
improved function is limited to the tissues and organs that have been
overloaded. Training the muscles of the legs, for example, does not improve
strength in the arms and shoulders.

In summary, these studies indicate that the risk of musculoskeletal
injury is directly proportional to the gap between one's accustomed level of
activity and the level of activity currently being performed. The larger the
overload the smaller the chance of adaptation and the greater the chance of
injury.

What Is the Relation Between Different Assigned Doses of Physical
Activity and Injury Among Persons of Similar Levels of Fitness or Physical
Activity Habits?

Assigning different physical activity regimens to groups of people who
are otherwise similar would provide experimental evidence about the relative
risks of different activity regimens. Although substantial numbers of clinical
trials with physical activity as an exposure have been done in recent years,
information about musculoskeletal injuries incurred during the trials and their
relation to dose of activity is sparse. Comparison among studies is difficult
because the assigned activity, outcome measures, period of study, and level of
detail about injuries differ markedly (Table G10.A1 [PDF - 215 KB] summarizes these
studies).

The amount and type of information has varied over time. Earlier reports
that included males aged 25 to 60 years, and used vigorous activity reported
high injury rates, nearly 50% (53-55). A few studies
specifically investigated the relationship between the dose of activity and the
incidence of injury (56-59) and, taken together, have
reported that the incidence of injury appears to be related to each of the
components of dose, frequency, duration, and intensity. More recent research
has focused on interventions to promote moderate intensity physical activity.
These studies suggest that the incidence of injuries attributable to
recommended levels of moderate intensity physical activity appear to be low (60-65), but their actual incidence, severity, and effect on
long-term physical activity participation are largely unknown. The study for
which injury rates for both intervention and control subjects appeared to be
most thoroughly reported found equivalent rates of musculoskeletal problems (64). A systematic review of interventions to prevent lower
limb soft tissue running injuries concluded "it is not possible to suggest an
optimal training load" (66), but that injuries are
associated with frequency, duration, intensity, or total amount of
training.

An observation noted in both early and more recent publications is the
apparent frequency of injuries during the first weeks of the intervention (53;67). One researcher advocated 8 to 10
weeks of preparative training before actually beginning a trial (53); another reported that half of all injuries occurred
within the first 4 weeks of a 24-week program (67).

In summary, reports from experimental studies suggest that frequency,
duration, and intensity all contribute to the risk of physical activity-related
adverse musculoskeletal events, that a substantial increase in activity level
leads to high rates of musculoskeletal problems, and that moderate intensity
physical activity appears to have low (but not precisely measured) injury
rates.

Are Injury Rates for Walking Less Than Injury Rates for Running?

On the surface, musculoskeletal injury rates from walking appear to be
less than injury rates for running. Annual injury rates among runners range
from 12% to 50% (median 35%) (2;3;12;25;27-29); for
walkers they range from 3% to 20% (2;12;16;17). A direct
comparison of injury rates for joggers and walkers reported rates of 75% and
54%, respectively, for all reported injuries, and 25% and 21%, respectively,
for injuries requiring 7 days of inactivity (59). In a
study of injury and adherence rates, the injury rate increased from 5% during
the 13 weeks of mostly walking to 57% during the weeks of mostly jogging (56).

The apparent differences in injury rates for walking and running deserve
closer scrutiny, however. Although the higher injury rates for running compared
with walking are generally assumed to be due to higher impact forces (68) and greater tension on muscles and tendons, some or all
of the difference may be due to the higher volume of activity performed by
runners. For example, the relative difference in injury risk between people
walking or running for recreation or sport is larger when the exposure is
minutes of participation (1.2 versus 3.6 injuries per 1,000 hours of
participation) than when the exposure is METminutes of participation (5.0
versus 6.0 injuries per 106 MET-minutes of participation) (Table G10.2) (12). In addition,
19% of persons expending 600 MET-minutes per week while walking were injured
compared with 21% of persons expending 700 MET-minutes per week while running
(Table G10.A2 [PDF - 136 KB] summarizes
these studies) (26). Data such as these suggest the risk
of injury from walking or running depends largely upon the total amount of
energy expended. At least 2 studies, however, have reported that the risk of
injury attributable to walking is seemingly independent of volume for
reasonable volumes (8;26;69). In one case, the volume of walking was largely
determined by walking more minutes per session (8;26), in the other by using inclined treadmills to raise the
intensity (69).

To summarize, it is not yet certain whether walking is intrinsically
safer than running or whether the reported lower rates of injury from walking
are because the total volume of activity (combination of duration, frequency,
and intensity) is less for walkers than for runners.

Question 3. Are Individuals at Increased
Risk of Sudden Adverse Cardiac Events When They Are Being Physically
Active?

Conclusions

During periods of vigorous physical activity all individuals, even
regularly active individuals, are at higher risk of sudden adverse cardiac
events (e.g., sudden death, myocardial infarction) than during periods when
they are being less active. However, active people are at less risk than
inactive people both during activity and during inactivity. When the risks
during activity and at rest are averaged over the whole day, active people have
a lower risk.

The risks of sudden adverse cardiac events are greater for those who
remain sedentary than for those who increase their regular level of physical
activity in a gradual manner. Risks of sudden cardiac adverse events are lower
for light- and moderate-intensity activities than for vigorous activities, and
likely depend on relative intensity as much or more than absolute intensity.

Because cardiovascular risks are more closely associated with intensity
of activity than with frequency or duration, common practice in aerobic
activity programs is to increase frequency and duration of activity before
increasing intensity. Although the safety of specific amounts of increase have
not been empirically established, the available scientific literature suggests
that adding a small and comfortable amount of light to moderate intensity
activity, such as walking, 5 to 15 minutes per session, 2 to 3 times per week,
to one's usual activities has a low risk of musculoskeletal injury and no known
risk of sudden severe cardiac events.

Current recommendations for physical activity state that asymptomatic
men and women who plan prudent increases to their daily physical activities do
not need to consult a health care provider before doing so. The incidence of
activity-related cardiovascular or musculoskeletal adverse events has not been
shown to be reduced by a medical consultation.

Rationale

As shown elsewhere in this report (see Part G. Section 1: All-Cause Mortality
and Part G. Section 2: Cardiorespiratory
Health), the cardiovascular benefits of regular physical
activity for adults outweigh the associated cardiovascular risks, including the
risk of sudden cardiac death and myocardial infarction. In addition, 7
epidemiologic studies published between 1984 and 2006 compared the incidence of
sudden adverse cardiac events (i.e., cardiac arrest, sudden death, myocardial
infarction) during or shortly after vigorous physical activity between less and
more active people (Table G10.A3
[PDF - 164 KB] summarizes these studies). These studies uniformly reported a lower
risk per minute of activity for more active people (Figure G10.4) (70-76).

Two of the studies also report that the absolute rate of sudden death
during vigorous physical activity is quite low (70;75).

In all 7 studies, the overall relative risk of vigorous physical
activity was elevated (median 4.9). When stratified by usual level of activity,
the median relative risk of adverse event during vigorous physical activity was
56 for the least active group in each study compared with a median relative
risk of 2.4 for the most active group.

Six of the studies used case-crossover methodology, a technique useful
for examining a brief period of risk related to a brief exposure, in which
cases serve as their own control. One study (74) was
exclusively a case control study; others augmented the case-crossover design
with case-control designs (72;76) or
performed the case-crossover analysis within a cohort design (70;75). In 6 of the studies, vigorous
physical activity was defined as requiring 6 or more METs; in one it was
defined as 5 or more METs (75).

All the studies included middle to older aged adults and, therefore,
provide no information about populations younger than 40 years of age. Two
studies included only males (70;74),
one included only females (75), and in the remainder, 23%
to 32% of the participants were female. One study reported that 90% of subjects
were white (71); others provided no information about the
racial composition of the study samples. Three studies excluded persons with
cardiovascular disease or other "major" chronic conditions (70;74;75).

Three studies provided information about the weighted or average risk of
sudden adverse cardiac events in addition to the relative risk per minute of
vigorous activity. Because active people are active for more minutes than are
inactive people, it is possible that the average risk of sudden adverse cardiac
events might be higher for them. Two studies reported that the average risk of
events declines with higher levels of regular physical activity (74;75); one reported no difference (70). Data from one of the studies shows that even though the
risk of the active person during activity exceeds the average risk of an
inactive person, the average risk of an active person over a 24-hour period is
less (Figure G10.5) (74;77). In the words of one investigator, to speak of sudden
death as a risk of exercise is misleading. Sudden death is, more accurately, a
risk of inactivity (78).

Two studies estimated the absolute incidence of sudden cardiac deaths,
reporting 3.5*10-6 and 3*10-8 per hour of vigorous
activity for men (70) and women (75),
respectively, confirming the low incidence of such adverse events.

What Do These Findings Imply for Light- or Moderate-Intensity Physical
Activity?

The previously described studies investigated risks associated with
vigorous physical activity (generally 6 or more METs). The cardiovascular risks
of light or moderate intensity physical activity are expected to be less, but
quantitative estimates are rare. One of the studies described above (76) reported the relative risk of mild-to-moderate activity
to be 0.9, essentially equivalent to the relative risk of sedentary activity
(1.1) and sleep (0.8). Among Finnish men, the risk of sudden cardiovascular
death during "non-strenuous" activity was reported to be about one-third the
risk during "strenuous" activity (79). In an evaluation of
an intervention promoting walking among healthy sedentary persons aged 70 to 89
years, abnormal heart rhythms were more common among subjects in the
intervention arm than among control subjects (64). The
intervention encouraged walking at moderate intensity with a goal of at least
150 minutes per week.

Figure G10.5. Risk of Cardiac Arrest During
Vigorous Activity and at Rest by Usual Level of Activity

Figure G10.5. Data Points

Risk by Level of Activity

Incidence of Cardiac Arrest
per
108 Hours at Risk

Average risk for sedentary person

18

Average risk for active person

5

Risk for active person when not active

4

Risk for active person when active

21

In summary, information about the activity-related cardiovascular risks
of moderate or light activity is limited. Available information is consistent
with the expectation that the risks are substantially lower than those
associated with vigorous activity. One would also expect the risks to be lower
for regularly active persons than sedentary persons, as is the case for
vigorous physical activity.

What Do These Findings Imply for Activities of Low Absolute but High
Relative (Perceived) Intensity?

These 7 studies examined the risks of activities requiring 6 or more
METs (in one case, 5 or more METs). Individuals, however, vary in their
capacity to perform activities with high energy requirements. In general,
capacity is lower for females than males, declines with age, appears to be
influenced by genetics, and is modulated by routine physical activity
practices. Activities requiring about 6 METs of energy expenditure generally
cannot be done by persons 80 years and older, are perceived as hard to very
hard for middle aged and older adults, and as moderately intense for young
adults (Table G10.4) (80).

Table G10.4. Absolute Intensity by Age
Group and Relative (Perceived) Intensity (80)

Relative
(Perceived)
Intensity

Absolute Intensity (METs) in Healthy
Adults

Young
(20-39 yr)

Absolute Intensity (METs) in Healthy
Adults

Middle-Aged
(40-64 yr)

Absolute Intensity (METs) in Healthy
Adults

Older Adults
(65-79 yr)

Absolute Intensity (METs) in Healthy
Adults

Oldest Adults
(80+ yr)

Very light

<2.4

<2.0

<1.6

≤1.0

Light

2.4-4.7

2.0-3.9

1.6-3.1

1.1-1.9

Moderate

4.8-7.1

4.0-5.9

3.2-4.7

2.0-2.9

Hard

7.2-10.1

6.0-8.4

4.8-6.7

3.0-4.25

Very hard

≥10.2

≥8.5

≥6.8

≥4.25

Maximal

12.0

10.0

8.0

5.0

MET, metabolic equivalent

To the extent that regular physical activity habits influence these
general findings, sedentary persons would find the relative intensity of any
activity of 6 METs to be higher than indicated by the table and very active
persons would find it to be lower. As a result, when performing the same
activity, sedentary individuals experience more cardiovascular stress than do
active individuals. This, plus the fact that sedentary individuals are probably
more likely to have atherosclerotic coronary arteries, helps explain the
elevated risk of sudden adverse cardiac events among sedentary individuals
during activity defined as vigorous on an absolute scale.

As a result, perceived intensity or level of exertion may be a better
indicator of cardiovascular stress than absolute intensity (5). Several scales of perceived exertion have been developed
and are in use. The terms used to describe a mid-range of exertion are "fairly
light" to "somewhat hard," "weak" to "moderate," or "light." Available evidence
suggests that individuals striving to be more physically active should,
especially initially, adjust their perceived effort to this light to moderate
level. They should not strive to perform at an arbitrary absolute level, such
as walking at a set pace of 3.5 or 4 miles per hour.

What Do These Findings Imply for Persons Interested in Increasing the
Amount or Intensity of Their Physical Activity Practices?

These findings, as did the research findings for the risk of
musculoskeletal injuries, indicate that the sedentary persons are most in need
of increasing their physical activity level and are those who are also at
greatest risk of adverse events when performing activities. The risk of sudden
adverse cardiac event is inversely related to the amount of vigorous physical
activity (Figure G10.6) (77).

Figure G10.6. Risk of Cardiac Arrest During
Activity and at Rest by Usual Level of Activity

Figure G10.6. Data Points

Risk by Level of Activity

Incidence of Cardiac Arrest
per
108 Hours at Risk

Risk for person with no minutes per week of vigorous physical
activity

18

Risk for person with 10 minutes per week of vigorous physical
activity while sedentary

13

Risk for person with 10 minutes per week of vigorous physical
activity while vigorously active

732

Risk for person with 80 minutes per week of vigorous physical
activity while sedentary

5

Risk for person with 80 minutes per week of vigorous physical
activity while vigorously active

66

Risk for person with 180 minutes per week of vigorous physical
activity while sedentary

4

Risk for person with 180 minutes per week of vigorous physical
activity while vigorously active

21

The risks of remaining sedentary, however, are greater than the risks of
becoming active, especially if increases in activity are prudent. For
cardiovascular risk, the intensity of the activity is of greatest concern (81). Because the cardiovascular risks are more closely
associated with intensity than with frequency or duration, common practice in
aerobic activity programs is to increase frequency and duration of activity
before increasing intensity (52). Although the safest
method of increasing one's physical activity has not been empirically
established, drawing upon the principle of overload and adaptation of exercise
science and the experience of supervisors of cardiac rehabilitation programs,
walking so that the perceived intensity is in the light to moderate range (5) until one is able to walk 20 to 30 minutes per session for
several weeks is an acceptable and safe way to begin. For older adults who are
increasing their physical activity level, cardiovascular adaptation to an
augmented activity regimen may take as long as 20 weeks or more (82), suggesting that activity levels should be increased at
monthly rather than weekly intervals.

Are There Any Types of Heart Problems or Other Medical Problems for
Which Vigorous Physical Activity, Even by Regularly Active Individuals, Poses
More Risk Than Benefit?

Unlike atherosclerotic coronary artery disease, for which regular
(including vigorous) physical activity is beneficial, hypertrophic
cardiomyopathy, anomalous coronary arteries, long QT syndrome, Marfan syndrome,
and other congenital cardiac anomalies are not benefited by regular, especially
vigorous, physical activity. The risks posed by these conditions rises with the
intensity of activity performed. Therefore, although regular moderate-intensity
physical activity may benefit individuals with these conditions by reducing the
risk of atherosclerotic disease, diabetes, obesity, and other conditions
related to under activity, vigorous physical should be avoided (81). People with sickle trait, a trait more common among
African Americans than other groups, are more likely to suffer rhabdomyolysis
and sudden death during exercise than those who do not carry the trait (83;84).

How Do These Studies Apply to Children, Adolescents, and Young Adults?

The studies reviewed for this chapter do not apply because children,
adolescents, and young adults rarely have atherosclerotic heart disease. In
contrast to adults for whom activity-related sudden adverse cardiac events is
almost always due to atherosclerotic coronary artery disease, activity-related
sudden adverse cardiac events among youth are primarily due to congenital
abnormalities.

Nontraumatic sports deaths (e.g., cardiovascular, hyperthermia,
rhabdomyolysis, and sickle cell trait) in high school and college athletes are
uncommon, with incidence rates of about 7.5 per million per year among male
athletes and 1.3 per million per year among female athletes (85). About 75% of the deaths are due to cardiovascular
causes. Because these deaths are so uncommon, the evaluation of various
screening mechanisms is difficult (86;87).

For children, adolescents, and young adults (ages 1 to 24 years) the
U.S. Preventive Services Task Force (USPSTF) recommends "counseling patients to
incorporate regular physical activity into their daily routines" (88). However, the USPSTF recommends against routine
electrocardiographic screening as part of the periodic or pre-participation
sports physical exam (88). Current recommendations of the
American Heart Association are that high school and collegiate athletes should
be evaluated by a "healthcare worker with the requisite training, medical
skills, and background," and that the evaluation "should include a complete
medical history and physical examination" including blood pressure measurement
(86). For a detailed discussion of issues related to
physical activity and youth, see Part G.
Section 9: Youth.

Is a Health Care Evaluation Necessary Before Augmenting One's Current
Physical Activity Practices?

Evidence that persons who consult with a health care provider before
increasing their physical activity receive more benefits or suffer fewer
adverse effects than persons who do not is not available. Also unknown is the
extent to which official recommendations to seek medical advice before
augmenting one's regular physical activity practices may reduce participation
in regular moderate physical activity by implying that being active may be less
safe and provide fewer benefits than being inactive (78).

Recent recommendations have suggested that asymptomatic men and women
who plan sensible increases in light to moderate physical activity do not need
to consult a health care provider before doing so (36;81). Others, generally concurring with the safety of small
increases in light to moderate activity, have recommended that "previously
inactive" men aged 40 years and older, women age 50 years and older, and people
who have chronic disease or risk factors for chronic disease should consult a
physician before starting a vigorous activity program (23;89;90).

These two perspectives, one calling attention to the safety of small
increases and the other to the risks of large increases, are consistent with
the findings of this chapter. A substantial increase in physical activity over
one's customary routine  either as a single episode, as shown in the
studies of acute adverse cardiac events, or as a more sustained program, as for
the military recruits  is associated with higher risks of adverse events
than are smaller and more gradual increases. The risk is associated with the
magnitude of the relative discrepancy between the usual level and new level of
activity rather than with other personal characteristics. Adding a small and
comfortable amount of walking, such as 5 to 15 minutes 2 to 3 times per week,
to one's usual daily activities has a low risk of musculoskeletal injury and no
known risk of sudden severe cardiac events. Choosing a comfortable level of
effort, initially increasing only frequency and duration of activities, and
allowing adequate time for adaptation to each new level of activity minimizes
the already low risk of injuries or other problems.

Question 4. What General Factors Influence
the Risks of Musculoskeletal Injury and Other Adverse Events Related to
Physical Activity?

Conclusions

In addition to the type, dose, and relative size of increase of physical
activity, the risk of musculoskeletal and other adverse events is related to
several key factors:

Demographic and personal characteristics. Low levels of physical
activity or fitness and previous musculoskeletal injuries are two of the most
important individual-level risk factors for injuries and other adverse
events.

Safety of the environment. Safe places for children to play,
structures that limit vehicular speed, and mechanisms that keep pedestrians and
bicyclists separated from motor vehicles reduce the risk of activity-associated
injuries. Fear of crime is a barrier to physical activity although no evidence
exists that people are at greater risk of crime during periods of activity than
periods of inactivity.

Prudent attention to each of these components of activity-related
risk can reduce the rate of adverse events and enlarge the benefit to risk
ratio.

Rationale

Demographic and Personal Characteristics

Demographic and personal characteristics influence the type and amount
of physical activity people do and, thereby, influence the risk of injury. This
section, however, is concerned not with whether these characteristics influence
the choice of activity but whether they influence directly the risk of injury.
Two types of questions may be of interest. First, do people with different
characteristics (e.g., old or young) but doing the same amount and type of
activity have the same risk of activity-related injury? Second, do people with
the same characteristic (e.g., overweight) but different activity habits (i.e.,
active or inactive) have the same risk of injuries from all causes? Information
is not always available to address both questions.

Age

Are older and younger people at the same risk of
activity-related injury? One would expect the risk of injuries
and other adverse events to increase with age. The physiologic changes of
aging, such as the decline in maximal heart rate and cardiac output, decline in
connective tissue elasticity, decline in balance, and fall in the speed of
reflexes, all would appear to make older people more easily afflicted by
physical activity-associated adverse events. Surprisingly few studies have
actually examined the topic. Studies of military recruits report higher injury
rates among older recruits even though the oldest recruits are in their mid to
upper 20s (35;91). Surveys of active
adults of all ages, however, report lower injury rates among older than younger
persons (9;12;14;17;92). This
surprising finding presumably arises from a confounding of age with exposure
 the fact that older individuals cannot perform and do not attempt to
perform at levels comparable to younger persons.

Are active and inactive people of similar age equally likely
to be injured? The incidence of injuries from all causes (not
only activity-related injuries) is a function of both age and level of physical
activity. Among younger people, physically active people report more injuries
requiring medical attention than inactive people; whereas among older people
inactive people report more injuries (Table G10.5)
(9).

 Report some leisure-time light-moderate or vigorous physical
activity but do not meet current recommendations

§ Report no leisure-time light-moderate or vigorous physical
activity

Are children and adolescents more susceptible to overuse
injuries? The growing child or adolescent may have an increased
susceptibility to certain repetitive stress injuries such as traction
apophysitis (e.g., Osgood-Schlatter disease), injuries to the immature spine,
or others (93). These established developmentally-linked
injuries and the growth of organized competitive sports for youth and the
sometimes prolonged and intensive training attendant with successful
participation in those sports has raised concern about "the sensibility and
safety of high-level athletics for any young person" (94).
Little empirical evidence is available on which to assess the magnitude and
risk factors for the problem. Drawing upon expert opinion and clinical
experience, recommendations, the American Academy of Pediatrics makes the
following recommendations (94;95):

Participation in sports should be at a level consistent with the
child's abilities and interests;

Athletes should take off 1 to 2 days per week and 2 to 3 months per
year from any specific sport;

Athletes should participate on only 1 team per season;

Athletes should not increase their training load by more than 10%
per week;

Coaches, trainers, and parents should be alert to the possibility of
overuse injuries;

Treatment recommendations should include alternatives to "rest only"
programs because they are not likely to be followed; and

Fun, sportsmanship, skill acquisition, and safety should be
emphasized.

Although overuse injuries may be a concern for some children and youth,
for the large majority of children insufficient physical activity is a greater
concern than too much activity (See Part G.
Section 9: Youth for a detailed discussion of this topic).

In summary, for a specific dose of activity older people are more likely
than younger people to be injured. In practice, however, older people
consciously or unconsciously appear to moderate their physical activity so that
they become injured less frequently than do younger persons. When compared to
inactive individuals, physically active younger persons are injured more
frequently than inactive younger persons whereas physically active older
persons are injured less frequently than inactive older persons.

Sex

Currently available research suggests that, aside from stress fractures
and injuries to the anterior cruciate ligament of the knee, which are more
common in females (51;96;97), males and females appear to be equally susceptible to
activity-related injuries (34;37;98-100). In population surveys, males are more likely to
report having been injured than females (34), but they are
also more likely to report being physically active (101).
In military studies in which males and females partake of the same dose of
activity, females are about twice as likely as males to be injured (10;102), but they, as a group, are also
less physically fit than males at the beginning of basic training (102;103). When injury rates are
adjusted for initial physical fitness, the risks of injury for females and
males are equivalent (98;102;103). This is consistent with
findings from college athletes in which sport- and sex-specific injury rates
are similar for males and females (99;100).

Race and Ethnicity

The influence of race and ethnicity on activity-related musculoskeletal
injuries has been uncommonly reported. Most studies of military recruits report
no differences among race and ethnic groups in the incidence of musculoskeletal
injuries (35;40-42). In one study,
the incidence of stress fractures was higher for whites than blacks (104). However, certain health conditions that are more
common in one race or ethnic group may influence injury or illness rates. For
instance, people with sickle trait, a trait more common among African Americans
than other groups, are more likely to suffer rhabdomyolysis and sudden death
during exercise than those who do not carry the trait (83;84).

Anatomical Characteristics

The few studies that have examined anatomical factors suggest that
self-reported leg-length discrepancies (105), and
clinically measured high arches (106), genu valgum (knock
knees) (107) and high quadriceps angle (107;108) are associated with higher
risks of musculoskeletal injuries than is the case for persons who do not have
these anatomical characteristics.

Behavioral Factors

Fitness and physical activity. Low levels of
physical fitness and physical activity are among the most important risk
factors for musculoskeletal injuries and sudden adverse cardiac events. Please
see the detailed discussion earlier in this chapter on the relationship between
physical activity, injuries, and levels of physical fitness.

Stretching. Persons with high or low levels of
flexibility appear to be more likely to be injured than persons in the middle
range of flexibility (35;40).
However, stretching, per se, has not been shown to be effective for either
injury prevention or improved performance (109). Some
evidence indicates that stretching combined with other actions, including warm
up, strength training, or general conditioning, prevents injuries (109).

Warming up and cooling down. Warming up and
cooling down before and after exercising are commonly recommended to prevent
injuries and adverse cardiac events. Although evidence is limited, various
combinations of warm-up, strength training, conditioning, and stretching are
associated with lower rates of musculoskeletal injuries (109).

Following a survey of major cardiovascular complications during exercise
training at cardiac rehabilitation programs, which reported that 44 (72%) of 61
adverse events occurred near the beginning or at the very end of the session
(110), careful warming up and cooling down have become
standard practice in cardiac rehabilitation programs. Evidence consistent with
myocardial ischemia during sudden strenuous exercise without warm-up has been
noted in some studies (111-113) but not others (114). In the period immediately after strenuous exercise
catecholamine blood levels are elevated, posing potential risk, especially for
persons with coronary artery disease (115).

Despite limited evidence of helpfulness, guidelines recommend 10 to 20
minutes of stretching and progressive warm-up activity before the main activity
session (116). Following the main activity, 10 to 20
minutes of gradually diminishing activity is recommended.

Existing Health and Medical Conditions and Behaviors

Tobacco use. Cigarette smoking (40;45;117) and
smokeless tobacco use (91) have been associated with
higher rates of musculoskeletal injuries among military recruits undergoing
basic training. The associations are strong (2- to 3-fold), are directly
related to the usual number of cigarettes smoked, and persist after controlling
for other risk factors, such as physical fitness.

Prior musculoskeletal injuries. Prior injury
is one of the most consistently reported and strongest risk factors for future
injury, with the risk generally reported to be about two-fold (10;29;34;35;41;47;108;118;119).
Reinjury may occur because the original injury has not healed or the wound and
surrounding structures have been inadequately rehabilitated (34), or because the primary risk factor has not been modified
(e.g., structural or training defect). Prior or current injury is one of the
most common barriers to participation in regular physical activity at
recommended levels (1-4).

Pregnancy. Current recommendations state that
participation in a wide range of aerobic and recreational activities of
moderate intensity is safe for healthy pregnant women with uncomplicated
pregnancies (120;121). Given the
many years, however, of clinical and public health efforts to promote and
provide appropriate medical care for pregnant women, surprisingly little is
considered firmly established about the benefits and risks to pregnant women
and their fetuses of physical activity. A systematic review by Kramer and
McDonald (122) noted that insufficient data from small
clinical trials whose methodological quality was not high made it difficult to
infer important risks or benefits of physical activity for the woman or the
fetus. Potential benefits specific to pregnancy, such as preventing gestational
diabetes and maternal weight control and promoting maternal fitness, easier and
less painful deliveries, and improved mental health, have been suggested by
epidemiologic studies but are not yet firmly established. (See Part G. Section 11. Understudied
Populations for a detailed discussion of physical activity
and pregnant women.) Prenatal medical evaluation is recommended for every
pregnant woman to be sure that she does not have one of the rare but absolute
contraindications for physical activity during pregnancy (120) and to develop an appropriate physical activity program
during the pregnancy and postpartum period.

Overweight and obesity. Studies of
activity-related injuries among military recruits, runners, and other active
people report varied findings. Some report no association (3;28) between weight status and risk of
activity-related injuries, and others report elevated rates among those with
higher body mass index (BMI) (118;123). Still others report elevated rates among military
personnel with lower and higher BMI (39;45) compared to those in the middle range.

Given the mixed findings and the fact that these groups contain few
overweight and almost no obese individuals, the results from these studies are
difficult to apply to overweight and obese individuals. For injuries of any
cause, not just activity-related injuries, population-wide studies indicate
that overweight and obese individuals are more likely than persons of normal
weight to report a medically attended injury (Table
G10.6) (9;124).

Table G10.6. Rate or Odds Ratio of
Medically Attended Injury of Any Cause, by BMI Category

Another way to look at this issue is to ask whether active and inactive
overweight people are at equal risk of activity-related injuries. One study has
found that obese persons who are regularly active have been reported to be
nearly 15% less likely to be injured from any cause than inactive obese persons
(9).

Acute upper respiratory tract infections
(URTI). Although evidence is not yet abundant, current scientific
literature suggests that people who are physically active at currently
recommended levels suffer fewer URTI than either sedentary individuals or
athletes participating in intense exercise training.

Recent studies of children (125), adolescents (126), adults (127), postmenopausal
women (128), and elderly adults (129) all report fewer URTI among more active individuals
than among less active. Another study among the elderly reported no difference
in the incidence of URTI but less fever and activity restriction among active
than inactive persons (130).

In contrast, recent studies that include persons participating in
intense exercise training such as elite athletes report either no difference
between groups (131;132) or the
highest rates of URTI among the athletes at the highest level of competition
(133-135). In studies with elite athletes the curves
showing infection rates across groups display a "J" shaped dose-response curve,
with the inactive participants having slightly elevated rates, moderately
active individuals the lowest rates, and the extremely active participants the
highest rates of URTI.

Engaging in moderately intense physical activity during a URTI appears
to be safe. Vigorous activity is discouraged, especially if evidence of
systemic involvement, such as fever, muscle aches, swollen lymph nodes, or
extreme fatigue, exists (136;137).

Chronic diseases. For many chronic diseases,
including the most prevalent conditions such as atherosclerotic heart disease,
diabetes, arthritis, or chronic lung disease, participation in an appropriately
designed physical activity program is therapeutically beneficial (138-141). Review and discussion of the evidence pertaining
to physical activity as a treatment for specific conditions is beyond the scope
of this chapter. For some severe conditions physical activity may be
contraindicated or severely restricted; the Canadian Society for Exercise
Physiology supported by Health Canada has proposed a list of such conditions
(142).

Current recommendations state that most people with a chronic disease
can safely add several minutes of walking or other light to moderate intensity
activity to their everyday activities. Persons with cardiovascular disease,
diabetes, or other active chronic conditions who want to begin engaging in
vigorous physical activity and who have not already developed
a physical activity plan with their health care provider may wish do so (36;81). Appropriately designed physical
activity regimens for individuals with a wide range of disabilities has been
shown to be safe and effective (see Part G. Section 11: Understudied
Populations, for a detailed discussion of physical activity
and people with disabilities).

Quality and Appropriateness of Equipment

Shoes

Comfortable shoes that fit properly are associated with less foot pain,
fewer blisters and ulcers, and lower risk of future development of foot
problems than are poorly fitting shoes. This is true for all people, regardless
of activity. Proper footwear is especially important for persons with diabetes
or other conditions that interfere with circulation or sensation of the feet.
Recent reviews indicate that shock absorbing inserts reduce the incidence of
stress fractures in military personnel (143) and that
external ankle supports reduce the incidence of ankle sprains during high-risk
activities such as basketball or soccer (144). The value
of pronation control and cushioning in running shoes, and lateral stability,
torsion control, traction, and cushioning in court shoes is generally assumed
despite limited scientific support (145). Comfort is one
of the, if not the, most important aspects of a sport shoe.

Clothing

For pedestrians and cyclists, the use of red, yellow, and orange
fluorescent materials improves recognition by drivers of vehicles, and lamps,
flashing lights, and reflective materials help at night (146). However, neither the use of fluorescent materials
during the day nor lights or reflective gear at night has been shown to reduce
collisions or injuries. Proper clothing also reduces the risk of injury from
cold or hot temperatures (see section on Climate, below) (147;148).

Bicycle Helmets

The protective effects of bicycle helmets have been firmly established,
and their use is recommended for commuting, recreational, and competitive
bicycling. A recent meta-analysis estimated that the use of bicycle helmets
reduced the risk of death by nearly 75%, risk of head injury and brain injury
by about 60%, and risk of facial injury about 50% (149).
An estimated 107,000 unnecessary bicycle-related head injuries, $81 million in
direct medical costs, and $2.3 billion in indirect health costs in 1997
resulted from bicyclists in the United States not wearing helmets (150).

Other Gear and Equipment

Proper protective equipment has been shown to reduce the rate of
specific types of injuries in a variety of activities. Breakaway bases and
reduced-impact balls in baseball and softball, mouth guards in basketball,
helmets in football, full face shields in hockey, wrist guards in in-line
skating, binding adjustments in skiing, and other protective gear have been
shown to reduce injuries (151).

Safety of the Environment

Physical activity is performed in many different environments, including
outdoor and indoor facilities, urban and rural settings, and hot and cold
temperatures. Many features of these environments influence the risk of
injuries. A common environment for physical activities such as walking,
gardening, bicycling, and playing is residential neighborhoods. Two-thirds
(66%) of respondents to a recent survey reported their neighborhood as a site
for physical activity (152). The next paragraphs pertain
to risk factors within residential neighborhoods, namely, traffic, crime, air
pollution, and weather.

Safety from Traffic and Crime

Traffic safety in neighborhoods. Injury rates
of pedestrians and cyclists in the United States are 2- to 4-fold higher than
in Germany and the Netherlands (153). Two important
features of safe walking and biking are lower traffic speed and separation from
traffic. Neighborhoods can be modified to reduce traffic speed and to assure
separation between pedestrians, cyclists, and motor vehicles. Mechanisms to
slow traffic, sometimes referred to as traffic calming, include 1) vertical
deflections (e.g., speed bumps), 2) horizontal deflections (e.g., bends), 3)
road narrowing, and 4) medians, four-way stops, and small roundabouts (154-156). Mechanisms to separate pedestrians and cyclists
from traffic include installation and maintenance of 1) sidewalks and bicycle
lanes, 2) pedestrian over- and underpasses, and 3) fences or parkways between
sidewalks and streets (155;157).
Methods of reducing the risk at crossings  areas shared by pedestrians,
bicyclists, and motor vehicles  include the installation of traffic
signals, pedestrian prompting devices (e.g., signs), in pavement flashing
lights to warn drivers when pedestrians are present, traffic signals with
exclusive walk signal phasing, refuge islands, raised medians, and improved
nighttime lighting (155;158;159). Less is known about creating
safe environments for bicycling than for walking (160).

Safe walking to school. When asked why their
children do not walk to school, 30% of parents cite traffic safety concerns and
12% cite fear of crime (161). Special programs have been
implemented specifically to improve the safety from both traffic and crime for
children going to and from school. The Federal Highway Administration's Safe
Routes to School provides funding to states for educational and environmental
change to encourage walking and biking to school (162).
An online newsletter reports that pedestrian injuries were not increased in
spite of more children walking and biking to school with Safe Routes to School
programs (163).

Crime reduction. Fear of violent crime is a
barrier to participation in physically active pursuits. Adults, adolescents,
and children who live in neighborhoods perceived to be unsafe have been shown
to be less physically active compared to those living in areas perceived to be
safe (164;165). No evidence exists
that people are at greater risk of violent crime (e.g., assault, rape) during
periods of activity than periods of inactivity. When considering walking,
running, or other physical activities in an unfamiliar place, crimes are less
likely to occur in places that are well lit, where other people are present,
and that are lacking signs of neglect such as litter, graffiti, empty
buildings, buildings in disrepair (e.g., broken windows) (154).

Safety tips to protect from traffic and crime are available (Table G10.7) (166). They have
face-validity although few have empirical support.

Air Pollution

The balance between the risks and benefits of being physically active in
air polluted at levels commonly experienced in the United States is not
established. Air pollution is a complex mixture of gases, liquids, and
particulate matter. Levels and constituents of air pollution vary among and
within cities. Air pollution is generally most intense near busy roadways and
industrial sites. It can be indoors or outdoors depending on the type and
origin of the pollutants.

Both long- and short-term exposure to air pollution have been shown to
increase all-cause, cardiovascular, and pulmonary mortality, hospital
admissions, emergency room visits, and symptoms (167;168). The Environmental Protection Agency has developed an
Air Quality Index and, depending upon the value of the index, individuals may
be advised to reduce or avoid "prolonged or heavy exertion" out of doors (169).

Physical activity increases the volume of inhaled air and exposure to
airborne toxins and allergens (170). Elevated levels of
air pollution are associated with reductions in maximal exercise performance
(171;172). Exercisers have been
warned to avoid exercising near heavy traffic and industrial sites, especially
during rush hour (170;173). Such
recommendations may be reasonable for individuals who can easily modify the
location or time for exercise. However, they may be a barrier to regular
physical activity for people with less flexible daily demands. In addition,
they take into account only short-term adverse effects of physical activity in
polluted air. The long-term benefits of regular physical activity in polluted
air may outweigh the short-term risks. Lower mortality rates among more active
than less active individuals in a polluted industrial community have been
reported (174). Regular physical activity in a polluted
environment may ameliorate the adverse effects of pollution just as it reduces
the adverse health effects of obesity and diabetes.

No evidence exists that physical activity in air polluted to the current
levels of American cities negates the benefits of physical activity. From a
health perspective, it would be preferable to reduce air pollution than reduce
the already low physical activity levels of Americans.

Temperature Extremes

It is safe for healthy people to be physically active in a wide range of
normally encountered temperatures. Proper clothing is important.
Acclimatization  physiologic adaptation (e.g., greater volume and less
concentrated sweat) to repeated exposures to warm weather  occurs over
several weeks and increases the safety warm weather activity. A protective
acclimatization to very cold weather does not occur. Specific guidelines
regarding activity in extremely cold (148) and hot (147) conditions have been published, as have guidelines for
maintaining adequate hydration in these settings (175).

Question 5. Do the Benefits of Regular
Physical Activity Outweigh the Risks?

Conclusions

The benefits of physical activity outweigh the risks.

Rationale

Outcomes that encompass a broad spectrum of medical maladies  such
as all-cause mortality, functional health, or medical expenditures  sum
both positive and negative effects of physical activity. All-cause mortality,
for example, encompasses deaths caused and prevented by activity; and medical
expenditures include costs incurred and avoided because of physical
activity.

Physically active people have lower all-cause mortality rates, higher
levels of functional health, and lower medical expenditures. (See
Part G. Section 1: All-Cause
Mortality and Part G.
Section 6: Functional Health.) Studies of the influence of
physical activity on medical expenditures consistently report lower costs for
active individuals when compared with inactive individuals. The savings may be
lower or even absent for younger persons, but for adults and older adults
physical activity is associated with lower medical costs. These medical
expenditures include costs associated with adverse effects and, therefore,
indicate an overall benefit from participation in regular physical
activity.

Are Medical Expenditures Lower for Physically Active People Than for
Inactive People?

Studies of the relationship between medical expenditures and physical
activity generally are of two types: direct comparisons of the expenditures of
more and less active people, and estimates of the excess costs incurred by
inactive people because they are more likely than active people to develop
certain conditions such as heart disease, diabetes, or colon cancer.

Direct Comparisons

Eight studies published in the last decade have compared direct medical
expenditures for active and inactive individuals who are members of the same
group or population, such as employees of a company, enrollees in a health
plan, or respondents to national surveys (see Table
G10.8 below. Table G10.A4
[PDF - 152 KB] also summarizes these studies) (176-183). All
report lower medical costs for active persons. For 7 studies, the reductions
ranged from 6% to 22% (median, 13%) (176;177;179-183); the eighth reported that
costs were reduced 4.7% for each day per week of physical activity (178). All minimized the confounding potential of chronic
diseases through exclusion, adjustment, or stratification. All adjusted their
findings for age and sex, 6 adjusted for BMI (177-181;183), and 4 adjusted for smoking (177-179;183). Subjects in all of the
studies were adults or older adults, with mean ages ranging from 45 years to 75
years.

Estimated Costs

A review summarizing the findings of 10 studies from 6 countries (US-3,
Canada-2, Holland-2, Australia-1, Switzerland-1, UK-1) of the annual excess
cost (in 2003 $US) per inactive person per year reports a range from -$109 to
$1305 (median $172) (184). These studies are done by
combining population-wide estimates of physical activity from national surveys
with estimates of relative risk of inactivity for specific diseases from
observational research studies. Four of the reviewed studies included both
direct and indirect costs; 6 included only direct costs. The one study
suggesting that inactive individuals have lower medical expenditures stratified
the data by age, reporting an annual cost saving of $109 per inactive person
aged 15 to 44 years compared with an expense of $87 per inactive person age 45
years and older. Similar methods have been used to develop estimates for 5
States (GA, MN, NY, SC, WA), with annual per capita direct medical costs
attributable to physical inactivity ranging from $19 (WA) to $79 (GA) (185).

Overall Summary and Conclusions

The benefits of regular physical activity outweigh the inherent risk of
adverse events. Still, adverse events are common and are an impediment to more
widespread participation in regular physical activity. Selection of low risk
activities and prudent behavior while doing any activity can minimize the
frequency and severity of adverse events and maximize the benefits of regular
physical activity.

The chapter has focused on musculoskeletal injuries, the most common
type of physical-activity associated adverse event, and sudden adverse cardiac
events, one of the most severe adverse events. The chapter also has emphasized
the key factors of any physical activity program, which are:

The type of activity;

The dose of activity as determined by the frequency, duration, and
intensity of participation; and

The rate of progression or change in the dose of activity.

Proper attention to each of these can substantially reduce the risk of
adverse events.

The overall conclusions of this chapter can be summarized as
follows:

Risk of adverse events and type of activity.
Activities with fewer and less forceful contacts with objects or other people
have appreciably lower injury rates than collision or contact sports. Walking
for exercise, gardening or yard work, bicycling or exercise cycling, dancing,
swimming, and golf, already popular in the United States, are activities with
the lowest injury rates.

Risk of adverse events and amount of activity.
The amount of physical activity is directly related to the risk of
musculoskeletal injury. Injury rates at the level of activity commonly
recommended (150 minutes per week of moderate intensity activity, or about 500
MET-minutes per week of activity) have been uncommonly documented but appear to
be low.

Risk of adverse events and change in amount of
activity. The risk of injury is directly related to size of
increase in the amount of physical activity performed. A series of small
increases in activity each followed by a period of adaptation is expected to
cause fewer injuries than larger increases. Adding a small and comfortable
amount of walking, such as 5 to 15 minutes 2 to 3 times per week, to one's
usual daily activities has a low risk of musculoskeletal injury and no known
risk of sudden severe cardiac events. Frequency and duration of aerobic
activity should be increased before intensity. Increases in activity level may
be made as often as weekly among youth, whereas monthly is more appropriate for
older or unfit adults. Attainment of the desired level of activity may require
a year or more, especially for elderly, obese, or habitually sedentary
individuals.

Risk of sudden adverse cardiac events. The
risk of sudden adverse cardiac events (e.g., sudden death, myocardial
infarction) are higher during periods of vigorous physical
activity than during periods of less intense activity or while at rest for all
individuals. However, active people are at less risk than inactive people both
during activity and during inactivity. When the risks during activity and at
rest are averaged over the whole day, active people have a lower average risk
than do inactive people.

The risks of sudden adverse cardiac events are greater for those who
remain sedentary than for those who increase their regular level of physical
activity, especially if the increase is gradual. Risks of sudden cardiac
adverse events are lower for light- and moderate-intensity activities, and
likely depend on relative intensity as much or more than absolute intensity.
The first changes in an aerobic activity program should be in the frequency and
duration rather than the intensity of the activity.

The information about activity-related musculoskeletal and
cardiovascular adverse events reviewed in this chapter is consistent with
previously made recommendations that asymptomatic men and women who plan
prudent increases to their daily physical activities do not need to consult a
health care provider before doing so. Evidence that persons who consult with a
health care provider before increasing their physical activity receive more
benefits or suffer fewer adverse events than persons who do not is not
available. Symptomatic persons or those with cardiovascular disease, diabetes,
or other active chronic conditions who want to begin engaging in vigorous
physical activity and who have not already developed a physical activity plan
with their health care provider may wish to do so.

Personal characteristics and behaviors associated with
higher risks of adverse events. Previous musculoskeletal injuries
and low levels of physical activity or fitness are two of the most important
individual level risk factors for injuries and other adverse events.

Other factors associated with higher risks of adverse
events. Proper equipment, such as bicycle helmets, reduces risk.
Neighborhood characteristics that limit vehicular speed and keep pedestrians
and bicyclists separated from motor vehicles also reduce the risk of
activity-associated injuries. Fear of crime is a barrier to physical activity
although there is no evidence that people are at greater risk of crime during
periods of activity than periods of inactivity.

Consistency of current findings with previous physical
activity recommendations. Adverse events have received relatively
little attention in previous physical activity recommendations. When mentioned,
however, the statements are consistent with the information presented in this
chapter. Several mention the importance of gradual increases in physical
activity (22-24;80;186), and several acknowledge the fact that injuries are a
potential barrier to participation in physical activity (23;24;80;187).

Research Needs

This review of the literature related to physical activity and adverse
events identified a number of questions that would benefit from additional
research:

Are active and inactive individuals at equal risk of
unintentional injuries? Although physically active individuals
are likely to incur more activity-related injuries, limited evidence suggests
that they may suffer fewer non-activity-related injuries and, therefore,
experience no more and maybe fewer overall injuries than inactive people. The
severity of injury and the type and amount of physical activity are likely to
be important determinants of the relationship.

What is the appropriate starting dose of activity for
individuals who have been inactive? Recommendations to
"gradually" increase one's physical activity are commonly made but there is
little information to support specific recommendations for an initial dose that
will maximize continued participation and minimize adverse events.

What are the appropriate size and frequency of increases in
physical activity? Recommendations to increase "gradually" are
common but there is little information to support specific recommendations.
Size and frequency of increase may also vary by age, length and severity of
inactivity, and other factors.

What are the incidence and risk factors for adverse events
associated with walking? Current literature suggests that risks
may be unrelated to either total volume of walking or intensity (using elevated
treadmills). Are these findings substantiated in other settings and
populations?

What are the most effective methods of reducing the
incidence and severity of adverse events among people who are increasing their
physical activity practices? The type, dose, rate of increase,
proper protective gear, and safety of the environment are major determinants of
injury risk and adherence to an activity regimen. Little is known about the
most effective methods of encouraging and enabling the public to safely
increase their physical activity practices.

Is there value, and if so for whom, to seeking advice from a
health care provider before increasing one's physical activity
practices? Although it does not address the preceding question
fully, one approach to reducing adverse events among persons who are increasing
their physical activity levels has been to suggest that some people (usually
those who are older or have one or more chronic conditions) should receive
permission and guidance from a health care provider before participating in
vigorous physical activity. The benefits and costs of such a suggestion are
unknown. Does a recommendation for people to develop an activity plan with a
health care provider prevent adverse events? Does it reduce participation in
physical activity? If both, what is the balance at the population level? Are
such recommendations justified for certain population subgroups? If so, which
ones?

[1] The amount of physical
activity has been measured and reported in various formats, including minutes
of exposure as in this study, miles or kilometers of running per week, or
kilocalories of energy expenditure. METminutes are being used with
increasing frequency. In this chapter, whenever feasible, an estimate of
exposure expressed in MET-minutes is provided. The estimate is less precise
than the original measures reported in this study but may be helpful when
comparing the findings from different studies. For this study, the categories
used by Parkkari and colleagues (12) did not correspond
exactly with the categories in the Compendium of Physical Activities (13), and approximations were made.

[2] The speed of running
is assumed to be 10 minutes per mile, an intensity of 10 METs.

American College of Sports Medicine Position
Stand. The recommended quantity and quality of exercise for developing and
maintaining cardiorespiratory and muscular fitness, and flexibility in healthy
adults. Med.Sci.Sports Exerc. 1998 Jun;30(6):975-91.

United States.Public Health Service.Office of
the Surgeon General., National Center for Chronic Disease Prevention and Health
Promotion (, President's Council on Physical Fitness and Sports (. Physical
activity and health : a report of the Surgeon General. Atlanta, Ga.;
[Washington, D.C.]; Pittsburgh, PA: U.S. Dept. of Health and Human Services,
Centers for Disease Control and Prevention, National Center for Chronic Disease
Prevention and Health Promotion ; President's Council on Physical Fitness and
Sports ; For sale by the Supt. of Docs.; 1996.